This invention is in the field of semiconductor devices and their fabrication and particularly relates to semiconductor devices fabricated with high Al-content III-V semiconductor materials.
The performance of semiconductor devices fabricated using high Al-content III-V semiconductor materials frequently degrades with time when operating in wet, high temperature environments. Some of this degradation may be due to the effects of high temperature on the semiconductor devices. The performance losses caused primarily by high temperature are not the focus of this description.
A large percentage of the degradation experienced by high Al-content III-V semiconductor materials in wet, high temperature environments is due to the formation of undesirable oxides of Al, primarily Al[OH].sub.3 or a solid solution containing Al[OH].sub.3 and other column III oxides. In the remainder of this description, all references to Al-hydroxide, Al-oxide hydroxide, Al oxide, and mixtures thereof include the possible presence of other column III elements formed from an alloy of these elements and Al.
Oxides which contain Al[OH].sub.3 as a major constituent have several pernicious traits. They are opaque and block the transmission of light from the semiconductor device. They also tend to attack the crystal structure of the device. The formation of Al[OH].sub.3 is particularly encouraged in those devices and in the portions of devices where photon flux is high, in a process called photon assisted hydrolization. In devices which do not produce light, the degradation process is similarly encouraged by carrier recombination, phonon generation, or any other energy generating process.
Preventing the formation of Al[OH].sub.3 in high Al-content III-V semiconductor materials is necessary to insure long operating lifetimes with acceptable performance in wet, high temperature environments. Several techniques are presently employed to minimize the degradation of high Al-content material. These include minimizing the thickness of the high Al-content layers and/or capping the material with lower Al-content material. The addition of In tends to stabilize high Al-content semiconductors against hydrolization degradation. The addition of In to the layer in question or capping the layer with In-bearing material may be beneficial. However, in some instances, it is not feasible, practical, convenient or cost efficient to utilize these techniques.
One other known method for preventing the formation of Al[OH].sub.3 is the formation of desirable oxides of Al over the high Al-content III-V materials to protect the materials from exposure to moisture and the resultant formation of Al[OH].sub.3. These oxides, herein called native oxides, are typically formed at higher temperatures and include Al(O)OH and Al.sub.2 O.sub.3. If these native oxides successfully prevent the formation of undesirable oxides such as Al[OH].sub.3 when the materials or devices using the materials operate in wet, high temperature environments, the materials and/or devices are considered to be passivated.
Within this description, a successfully passivated device is one that has survived a wet, high-temperature operating life ("WHTOL") test under active operation without significant device degradation for 1,000 hours. For example, if a light emitting diode ("LED") undergoes a WHTOL test with less than a 20% reduction in emitted light, exclusive of the degradation caused solely by the elevated temperatures, it is passivated for purposes of this invention. Herein, the conditions of a WHTOL test are under active operation (e.g. forward bias in an LED) in an atmosphere of 85% relative humidity and a temperature of 85.degree. C. Although these conditions are extreme, they provide a good indication of how devices will perform in hot and high humidity conditions.
Although many methods for forming native oxides from an Al-bearing group III-V semiconductor materials are known, one in particular forms a high quality oxide. This method is described in U.S. Pat. No. 5,262,360, issued on Nov. 16, 1993 to Holonyak, Jr. et al., entitled "AlGaAs Native Oxide" ("Holonyak") and is applicable herein. Holonyak describes the use of a water-vapor environment at elevated temperatures to form the native oxide. The Holonyak process produces a strong and durable native oxide.
Forming a native oxide on an Al-bearing semiconductor layer in a device is not necessarily sufficient to passivate the device. Many process and material variables must be properly controlled to use the Holonyak process to passivate LEDs and other semiconductor devices successfully.